The present invention relates to a beamformer and more particularly to a device that forms beams by electrically delaying the signals from a plurality of hydrophones and then summing the delayed signals.
Implementation of sonar systems which have been designed and built for various applications has been accomplished with many different technologies and in many different configurations. In most applications, however, the signal from each element in the sonar receiving array must be delayed appropriately. The delay is used to eliminate the phase differences caused by differences in signal propagation time from signal source to individual hydrophone elements. After the delay, the signals, often shaded (weighted) to improve array beam characteristics (side lobe reduction) from each hydrophone are then added together. The beamformer of a sonar system is the unit which delays, shades, and sums the individual signals to form beams. The beamforming process is done either with time delays or with phase shifting so that signals from hydrophones are in phase with each other prior to their addition. For narrowband beamforming applications, a phase shift technique (phased array beamforming) is used to align the signals from each hydrophone. For broadband beamformers, the phase change over a broad frequency range is too great to be compensated for by phased array beamforming without degrading the performance. Thus, time delay beamforming is used in broadband systems.
Time domain beamforming in the conventional sense is achieved by delaying each array's hydrophone output signal (shaded if so required) by an appropriate time delay and then adding them. The sensor outputs are typically weighted (shaded) so that the beam's spatial response can be improved. In a time domain beamformer, time delays for each element are generally implemented in one of two ways. The first method is to delay each signal the appropriate amount and then sum all the signals. The second alternative is first to sum all incoming signals with some required time, delay the partial sums an incremental amount and sum them with the signals having the next longest delay, and then delay this composite signal the correct amount and sum it with signals from additional elements.
Originally, sonar technology was developed using analog devices. The beamformers built for the earliest sonars used analog delay lines and analog processing to implement the sum and delay type of beamformer. Networks of resistors were used to sum appropriate stave signals into delay line taps, and then beams were formed from the delayed signals at the output of each delay line. The earliest designs were refined by using capacitors instead of resistors at some points to improve response characteristics of the delay line. Further refinements included the addition of mechanical switches or capacitive plates to change the selection of the input elements. The number of beams that can be implemented easily is limited with either of these designs since each beam requires many delay lines. As a result, early beamformers often used scanning switches to sequentially look in all directions; however, performance was compromised. Only one beam was available at a given time. If the scanning rate was high, then the signals were degraded.
Only recently has the electronics industry provided the technology and the capability to build practical digital sonar systems. One of the earliest digital techniques applied to the problem of processing the output of an array of acoustic hydrophones was the digital multibeam steering system (DIMUS). In this system the signal from each element is filtered, equalized, and clipped prior to beamforming. Then digital shift registers are used for time delay so that the acoustic arrays may be steered. The output of each element is reduced to a sequence of polarity samples (1-bit data) and delayed by an integral multiple of the sampling period. Then the delayed sequences for all elements are added, squared, and smoothed.
The DIMUS system was first developed when digital hardware was in its infancy. Consequently there are some disadvantages in this system due to the requirement that the incoming signal be carefully equalized in amplitude prior to clipping. If this is not done, a single, strong frequency component will override all other components and cause considerable degradation of the beamformer. Furthermore, when a narrowband directional tone is present, it masks out all other signals. Because of this feature, it is easily jammed with a narrowband signal.
In order to have good sidelobe suppression and wide dynamic range in a digital system, line signal amplitude quantization (more than one bit) is required. By increasing the number of quantization levels, the requirements for data lines, storage media, sampling modules, and arithmetic processing are increased. Use of quadrature sampling allows minimization of sampling and processing requirements and the use of geometric representation of signals allows use of reasonable word sizes at the beamformer input.
The performance of the DIMUS type of beamformer can be greatly enhanced by replacing the clipper with an analog-to-digital (A/D) converter which thus provides information about the signal expressed in more than one bit words. When quantization with more bits is done, the dynamic range of the system is determined by the number of bits and capability will be improved substantially. However, the improvements have a practical limit. Electronic circuitry for this type of beamformer can become very complex as the number of quantization bits becomes large, because each signal has a number of lines equal to the number of associated bits. Improvements to the DIMUS beamformer can be made by using random access memory (RAM) instead of shift registers. This approach simplifies the electronics but places stringent speed requirements on the RAM. Consequently this type of system would be difficult to implement at this time in any practical applications.
A hybrid technique employing a discrete time, continuous voltage approach is to use a charge coupled device (CCD) for the delay function in delay and sum beamformers. The main advantages are that high speed A/D converters and memories are not needed and multiple beams can be formed in one device. The disadvantage of this approach is that the electronics are still complicated and the dynamic range of the beamformer is limited by the charge coupled device.
Another approach is to use high speed digital systems in which beamforming and scanning are done concurrently. In general, high speed digital scanning is practical only in narrowband systems where phase shift beamforming can best be used. Because only one cycle of phase shift is required to form a beam, phase shift networks can be used to shorten the length of the delay line. The main advantage of the phase shift approach is that it requires less hardware to implement and, therefore, costs less. A combination of digital phase shift beamforming with circular array geometry to provide a scanned beam output for narrowband waveforms has been shown to provide good dynamic range using minimal hardware.